1. Field of the Invention
The present invention relates to a pulse wave sensor adapted to be pressed on a body surface of a living subject for detecting a pressure pulse wave produced from an artery of the subject.
2. Discussion of the Related Art
There is known a pulse wave sensor having a press surface adapted to be pressed on a body surface of a living subject, a protruding portion protruding from the press surface, and sensing means provided in a top surface of the protruding portion, for detecting a pressure pulse wave produced from an artery of the subject. As disclosed in U.S. Pat. No. 5,179,956 assigned to the Assignee of the present application and Laid-open Publication No. 4-67839 of unexamined Japanese Patent Application, the pulse wave sensor as described above has a resilient protective layer formed of an epoxy resin, silicone resin or silicone rubber. More specifically described, the resilient protective layer covers the press surface of the protruding portion adapted to be pressed on the body surface and additionally forms inclined surfaces between the protruding portion and a protection plate, which surfaces are inclined from the press surface of the protruding portion toward the protection plate. This resilient protective layer is provided for the purpose of protecting one or more pressure sensing elements provided in the top surface of the protruding portion and a cable connected thereto, as well as mitigating pain as felt by the subject when the edges and/or corners of the protruding portion are pressed on a skin (i.e., body surface) of the subject for detecting the pulse wave. In measuring blood pressure of the subject by using this pulse wave sensor, the protruding portion of the pulse wave sensor is pressed on the body surface over an artery such as a radial artery from which the pulse wave is detected, such that the artery partially becomes flattened. As a result, pressure transmitted from the artery is detected by the pressure sensing elements of the pulse wave sensor.
In many cases, however, hard bone and tendon are present near the artery from which the pressure pulse wave is to be detected. When the pulse wave sensor is pressed on the body surface with a pressing force necessary to measure the blood pressure, the resilient protective layer which surrounds the protruding portion receives relatively large pressure from the bone and tendon, and tensile force caused by friction between the protruding portion of the pulse wave sensor and the skin of the subject on which the pulse wave sensor is pressed. The pressure from the bone and tendon and the tensile force, each received by the resilient protecting layer, are undesirably transmitted to the top surface of the protruding portion covered with the resilient protective layer, and consequently adversely increase the pressure detected by the pressure sensing element. Therefore, the blood pressure detected by the pulse wave sensor as described above inevitably includes an erroneous component which results from the above pressure and tensile force. Thus, the above-described conventional pulse wave sensor is not capable of assuring accurate measurement of the blood pressure.